CANBERRA (Bloomberg) - Former Australian Prime Minister John Howard, honoured in the United States for his backing of efforts to tackle global terrorism, says he believes billionaire presidential candidate Donald Trump is too unstable to hold office.

"My natural inclination would be to see a Republican in the White House, but I tremble at the thought of Trump being president," Mr Howard told Sky News television in an interview on Sunday (Feb 28). "There's an instability about him that bothers me."

Mr Howard, who won four elections and served in office from 1996 to 2007 to become Australia's second-longest serving prime minister, said that despite his reservations over Mr Trump's sometimes "brusque" demeanor, the contender's popularity was understandable.

"He's doing well because he's saying things that a lot of people think should be said, that the current political class aren't willing to say," said Mr Howard, who was awarded the US Presidential Medal of Freedom by then President George W. Bush in 2009.

"In part, his success is emblematic of people's frustration with political correctness."

Florida Senator Marco Rubio would be better placed to challenge Mrs Hillary Clinton in an national election, should the two candidates win their respective party nominations, Mr Howard said.